Twin-screw marine drive mechanism for relatively small ship



Dec. 10, 1963 A. E. FRANK ETAL 3,113,549

TWIN-SCREW MARINE DRIVE MECHANISM FOR RELATIVELY SMALL SHIP Filed Jan.15, 1962 6 Sheets-Sheet l 1a 1+ 11 1 Z1 19 4-.P 22 I. l

4 INVENTORS AUGUST 5. FRANK F a 2 JAMES L. snow/v BY ATTORNEY TWIN-SCREWMARINE DRIVE MECHANISM FOR RELATIVELY SMALL SHIP Filed Jan. 15, 1962 6Sheets-Sheet 2 INVENTORS AUGUST E. FPAIVK JAMES L. BROWN ATTORNEY Dec.10, 1963 A. E. FRANK ETAL 4 TWIN-SCREW MARINE DRIVE MECHANISM FORRELATIVELY SMALL Filed Jan. 15, 1962 6 Sheets-Sheet 3 INVENTORS AUGUSTE. FRANK JAMES L. BROWN ATTORNEY TWIN-SCREW MARINE DRIVE MECHANISM FORRELATIVELY SMALL SHIP Filed Jan. 15, 1962 6 Sheets-Sheet 4 IN V EN TORSAUGUST E. FRANK JAMES L. BROWN ATTORNEY Dec. 10, 1963 A. E. FRANK ETAL3,113,549

TWIN-SCREW MARINE DRIVE MECHANISM FOR RELATIVELY SMALL SHIP Filed Jan.15., 1962 6 Sheets-Sheet 5 INVENTORS AUGUST E. FRANK JAMES L. BROWNATTORNEY ec. 1963 A. E. FRANK ETAL- 3,113,549

TWIN-SCREW MARINE DRIVE MECHANISM FOR RELATIVELY SMALL SHIP Filed Jan.15; 1962 6 Sheets-Sheet 6 mrzmans 1/6. 7 5551322 15 bi sv'v l ATTORNEY.United States Patent Office a Dec, 10,1963

The present invention relates to a twin-screw marine drive mechanism foruse with a single inboard engine, and more particularly, to such a drivemechanism which is especially suitable for relatively-small cabincruisers.

Heretofore, in the prior art of which we are aware, twin-screwoppositely-rotating propellers have long been used for marinepropulsion, the basic advantages generally being increased power,balanced thrust, and greater safety. However, such mechanisms suiferfrom a number of disadvantages, the most salient of which are asfollows: First, it has long been the practice to provide separateengines, one for each of the propellers; and inasmuch as it is difficultto synchronize the engine speeds to close limits, one of the propellersinevitably turns somewhat faster than its counterpart, with the resultthat the ship experiences an undesiriable degree of slippage. Secondly,the cost of installation and maintenance of multi-screw or twin-screwmarine propulsion systems has heretofore been prohibitive, with theresult that the use of such systems has generally been confined to onlythe very large and luxurious craft. Thirdly, the complexity of prior arttwin-screw installations has necessitated a high degree of navigationalskill and has tended to preclude an easy and efifective control over thecraft. Fourthly, the structural composition of prior art twin-screwsystems has required special components and extensive maintenance andcare, with the result that the operational and maintenance costs havebeen excessive.

Accordingly, it is the paramount object of the present invention toalleviate the aforementioned deficiencies by providing a twin-screwmarine drive mechanism coupled to a single inboard engine, and utilizedfor relativelysmall craft in the order of approximately thirty feet orless, whereby all of the basic advantages of twin-screw dual-engineships are realized at a cost consonant with the ownership andmaintenance of smaller craft.

It is another object of the present invention to provide, for use inconjunction with a single inboard engine, a twin-screw marine drivemechanism having increased safety and superior handling qualities.

It is yet another object of the present invention to provide atwin-screw marine drive mechanism for a relativelysmall craft, whereinone of the propellors may be placed in forward drive and the other inreverse, whereby the craft will turn in a circle whose diameter isapproximately equal to the length of the craft.

It is still another object of the present invention to provide, in arelatively-small craft having a single inboard engine, a twin-screwmarine drive mechanism that facilitates superior maneuverability, easieroperation, and more effective operator control.

It is a further object of the present invention to provide, for use witha single inboard engine, a twin-screw marine drive mechanism having aseparably-operable clutch mechanism in each of the twin-screw drivesystems, wherein the control over each of the propellers is separate andindependent from each other, and wherein each of the clutch mechanismshas synchronization means to prevent a clashing of clutch surfaceswhenever the mechanism is shifted.

It is yet a further object of the present invention to provide, for useon a small craft having a single inboard engine, a twin-screw marinedrive mechanism that is relatively-inexpensive, uses standard componentsthat are readily available,is easy to install on either new or existingcraft, and is simple to maintain.

It is a yet still further object of the present invention to provide atwin-screw marine drive mechanism that may be mass-produced easily andeconomically.

It is still yet another object of the present invention to provide foruse on a relatively-small craft, a marine drive mechanism whosepropellers are in the water all of the time, thus achieving lesscavitation than that which would ordinarly be experienced with anoutboard motor used on a craft of the same size.

These and other objects of the present invention will become apparentfrom a reading of the following specification, taken in conjunction withthe enclosed drawings, in which:

FIGURE 1 is a pictorial view of a relatively-small craft of the cabincruiser type in the order of approximately thirty feet or less, withpart of the stern being broken away to illustrate the twin-screw marinedrive mechanism coupled to a single inboard engine;

FIGURE 2 is an enlarged view taken on the line 2-2 of FIGURE 1;

FIGURE 3 is an enlarged view taken along the lines 3-3 of FIGURE 2,partly in elevation and partly in section, showing the main drivespindle, one of the individual propellers, one of the clutch mechanismsfor controlling the drive to the individual propeller, and the means forshifting the propellers (in unison) and angularly with respect to keelof the boat;

FIGURE 4 taken along the lines 44 of FIGURE 2, partly in elevation andpartly in section, and showing the further details of the individualdrive mechanism for each propeller;

FIGURE 5 is a view taken along the lines 5-5 of FIGURE 4, showing thefurther details of one of the clutch mechanisms;

FIGURE 6 is a view taken along the lines 66 of FIGURE 2, showing themeans for coupling each of the propellers to the single inboard engine;and

FIGURE 7 is an exploded view of one of the clutch mechanisms.

With reference to FIGURE 1, there is illustrated a relatively small ship10 of the cabin cruiser variety, in the order of thirty feet or less,wherein the teachings of the present invention may find moreparticularly utility. Such a ship 10 includes a hull 11, keel 12, howportion 13, stern portion 14, top deck 15, and cabin 16; and as furtherillustrated in FIGURES 1 and 2, the ship 10 also includes a lower deck17, a single inboard engine 18 mounted on the lower deck 17, a maindrive spindle 19 in the stern portion 14 of the ship 10, the spindle 19being disposed transversely of the keel 12, drive means 26 from theengine 18 to the spindle 19, and propeller drive housings 21 and 22 forthe respective propellers 23 and 24. Moreover, single control meansincluding the steering wheel 25 is provided for simultaneously adjustingthe angular position of the propellers 23 and 24 with respect to thekeel 12, while a pair of clutch control levers 26a and 26b are alsoprovided for selectively actuating either of the propellers Z3 and 24,respectively, for rotation in forward, neutral, or reverse.

With reference to FIGURE 3, the single inboard engine 18, which forconvenience may comprise a 275 H.P. Chrysler inboard marine engine,includes the motor shaft 27, which is journaled in tapered rollerbearings 28 and 29, the latter being retained, respectively, in housingmembers 30 and 31. Member 31 is in turn supported by housing member 32,while member 30 is bolted to the housing for the drive means 2% Aremovable top cover 33 is fitted over the housing members 30 and 31,thus providing for lubrication and easy maintenance of the rollerbearings 28 and 29. A worm wheel 34 is keyed to shaft 27 and engages acorresponding worm wheel 35 carried by the main drive spindle 19, asshown in FIG- URES 3 and 6. The main drive spindle 19 is journaled intapered roller bearings 36 and 37 retained, respectively, in housingmembers 38 and 39. The main drive spindle 19 projects into each of thepropeller drive housings 21 and 22 on either side of the drive means 26;and each of the propeller drive housings 21 and 22, and the clutchmechanisms contained therein, is identical. Hence, only one of theclutch mechanisms housed in the propeller drive housings 21 and 22 willbe explained in detail.

With reference to FIGURE 3, the main drive spindle 19 is journaled inpropeller drive housing 21 by means of a sleeve bearing 33, which is inturn supported by a housing member 39. A pair of oppositely-rotatingbevel gears 46 and 41 are loosely mounted upon spindle 19, andrespective bushings 42 and 43 are interposed between the bevel gears 40and 41 and the spindle 19. Another housing member 44 is secured to aside wall 45 of the propeller drive housing 21, and housing member 44has a washer 46 doweled thereto, the washer 46 being intermediately ofthe side wall 45 and the hub of bevel gear 40.

With reference to FIGURES 3, 4, 5, and 7, a clutch mechanism 47 isprovided for mechanically coupling either of the bevel gears 40 and 41to the drive spindle 19, thus facilitating a reversal in the directionof rotation of the propeller 23, as may be desired by the operator. Themain drive spindle 19 has an intermediate portion 48 that is milled toacquire a square cross-section, or its equiva lent, and a pair of firstclutch elements 49 and 50 are keyed to the intermediate portion 48 ofspindle 19 for rotation in unison. The first clutch elements 49 and 50are split by an axial plane and are identical to each other; and whenjoined together, the elements 49 and 50 have a square cross-sectionedbore 51, which cooperates with or is keyed to the intermediate portion48 of the main drive spindle 19. The outer cylindrical surface ofelements 49 and 50 is provided with a series of longitudinal splines 52circumferentially-spaced one from another. A second pair of clutchelements 53 and 54 is provided. Elements 53 and 54 are identical to eachother and are split transversely with respect to each other and withrespect to the axis of spindle 19; and a compression spring 55 isinterposed therebetween. Each of the second clutch elements 53 and 54 isprovided with a series of internal longitudinal splines 56,circumferentially spaced one from another, and adapted to engage theouter splines 52 of the first clutch elements 49 and 50. Thus therotational drive is transmitted from the main drive spindle 19 via thefirst clutch elements 49 and 59 to the second clutch elements 53 and 54.Moreover, the second clutch elements 53 and 54 each have respectivecollar portions 57 and 58, with the spring 55 being received incorresponding internal annular recesses 59, 60 formed respectively inthe collars 57, 58. Furthermore, the collars 57, 58 are formed with aseries of circumferentially-shaped splines adapted to engagecorresponding internal splines formed on an axially-shiftable outerclutch element 61. Outer clutch element 61 has an external annulargroove 62 adapted to receive rollers 63 and 64 (see FIGURE of a shiftingmeans hereinafter to be described. Thus, the outer clutch element 61 maybe axially mowed or adjusted along the second clutch elements 53, 54.Moreover, outer clutch element 61 has a pair of axially-protrudingclutch teeth 65 on one side, and a corresponding pair of clutch teeth 66on the other side, which are adapted to engage corresponding clutchteeth 67 and 68 formed, respectively, in the internal faces of bevelgears 40, 41. Consequently, outer clutch element 61 may be axiallyshifted so as to engage the clutch teeth 65 with the corresponding teeth67 on bevel gear 40, or else, the clutch teeth 66 with the correspondingteeth 63 of bevel gear 41, thus coupling either of the gears 40, 41 tothe main drive spindle 19. One of the gears 40, 41, will correspond tothe forward position of the clutch 4 mechanism 47, while the other ofthe gears 40, 41 will correspond to the reverse" position of the clutchmechanism 47, and naturally, of the propeller 23 or 24; and moreover,the position of the outer clutch element 61, intermediate of the bevelgears 46, 41, corresponds, then, to a neutral position (as isillustrated in FIGURE 3), wherein no drive is being transmitted to thepropeller 23.

Furthermore, the primary purpose of the second clutch elements 53, 54,and of the spring 55 interposed therebetween, is to prevent anyundesirable clashing of the corresponding clutch teeth, 65 with 67, or66 with 68, whenever the outer clutch element 61 is axially shifted fromthe forward to the reverse position, or vice-versa. The spring 55 forcesthe second clutch elements 53, 54 to bear against the respective endfaces of the bushings 42, 43; and as a result, whenever the outer clutchelement 61 is being shifted, the frictional drag created by either ofthe second clutch elements (53 or 54) against the respective bushing (42or 43) of the opposite bevel gear, will cause that particular gear(which is to be subsequently engaged) to slow down sulficiently to allowthe teeth on the outer clutch element 61 to make an easy and positiveengagement without any excessive impacting or mashing or ratchetingtherebetween. The use of the bushings 42, 43, as part of the means toachieve the aforesaid braking action of the desired gear to be engaged,is more desirable than having the second clutch elements 53, 54otherwise create a frictional drag upon the respective internal faces ofthe bevel gears 46, 41 intermediate of the teeth 67 or 68; replacementof the bushings 42, 43 is simple and inexpensive, whereas replacement ofthe bevel gears 40, 41 would be more expensive. Hence, the bushings 42,43 have a two-fold purpose: one, to journal the bevel gears 46, 41 uponthe main drive spindle 19; and two, to cooperate with the second clutchelements '53, 54 to facilitate a synchronization and convenientengagement of the clutch teeth (for example, 65 with 67), yet providefor a simple and inexpensive replacement of the bushings 42, 43 afterexperiencing a lengthy period of use.

Also, the second clutch elements 53, 54 are retained within the outerclutch element 61 by means of plurality split washers, one of which isdenoted as at 69, and which are secured to the outer clutch element 61by means of screws 70 received in corresponding tapped holes 71 formedin the outer clutch element 61.

The outer clutch element 61 is axially shifted in the following manner:The rollers 63 and 64, which are received in the external annular groove62 of the outer clutch element 61, are carried by a pair of dependingarms 72 and 73, respectively. Each of the arms 72 and 73 isintegrally-formed with a respective sleeve 74, which, by means of akeyway 75 and set screw 76, is keyed to a shifter shaft 77. Shiftershaft 77 is journaled in the housing 21 and is adapted to have a limitedrotation or pivoting movement about its axis (by manual or powerdrivenmeans known to one skilled in the art) in order to pivot the dependingarms 72 and 73 about the axis of shaft 77 and hence cause the rollers 63and 64 to axially slide the outer clutch element 61 along the splinedcollars 57, 58 of the second clutch elements 53, 54, respectively.

Moreover, the shifter shaft 77 has a protruding portion 78 having flatsformed thereon so as to facilitate a manual shifting, which is to say,pivoting, of the shaft 77; but it will be appreciated by those skilledin the art, that the use of a power-driven mechanism to pivotably shiftthe shifter shaft 77 is within the scope of the present invention, andindeed, for this very purpose, the mechanism (denoted generally as at79) is illustrated herein.

In such a. manner, the main drive spindle 19 may be mechanically coupledto the respective bevel gear 40 or 41 in order to drive the respectivepropeller, say propeller 23, on the forward or reverse direction; andthe means for coupling the bevel gears 40, 41 to the propeller 23 willnow be explained in detail.

With reference, again, to FIGURES 3 and 4, each of the bevel gears 40and 41 as rotating oppositely from the other and is constantly beingrotated, although it will be understood that the drive is beingtransmitted, it at all, through only one of the bevel gears 40 or 41,depending upon the position of the clutch mechanism 47. The bevel gears40 and 41 are constantly in mesh with an intermediate bevel gear 80,which is coupled to a vertical intermediate spindle 81 by means ofsplines 82. Spindle 81 is journaled in a ball bearing 83, and the end ofspindle 81 carries another bevel gear 84. Bevel gear 84 meshes with acorresponding bevel gear 85 which is carried by the propeller spindle86. The propeller spindle 86 is journaled on ball bearings 87 and 88,and the propeller 23 is retained on the propeller spindle 86 by means ofa lock nut 89. A resilient hub 90 comprising a sleeve (of rubber orsimilar material) is loosely interposed between the propeller 23 and thepropeller spindle 86 and is forced against a suitable shoulder on thespindle by the lock nut 89. The resilient hub 90, in combination withthe lock nut 89, forms a slip clutch between the propeller 23 and thepropeller spindle 86. The purpose of this slip clutch or slip couplingis as follows: one, to preclude the pos sibility of mechanical componentdamage should the propeller 23 strike an underwater object, and two, toreduce any torsional or shock loads from being transmitted through thedrive mechanism. A stabilizer 91 is provided. The stabilizer 91precludes any underwater objects from striking an unduly hard blow uponthe propeller 23; and also, the stabilizer 91 is utilized in thesteering of the craft 10, as will hereinafter be explained in detail.Moreover, the end of the propeller spindle 89 (opposite from the locknut 89) is supported and journaled in another ball bearing 92, which isretained in the forwardmost portion of the stabilizer 91.

The means for angularly shifting or pivoting the propeller 23 andstabilizer 91 will now be explained with reference, again, to FIGURES 3and 4. Intermediate bevel gear 80 is journaled in a bushing 93, which ispress-fitted within the end of an inner vertical tube 94. An outervertical tube 95 is provided concentrically with the inner vertical tube94, there being a sleeve 96 interposed therebetween. The opposite end ofthe inner vertical sleeve 94 is threadably received within a bore 97(formed at the top of the stabilizer 91) and is locked against rotationby means of a screw 98. Screw 98 also is received within an innerconcentric member 99, and

a bushing 100 is interposed between the member 99 and the verticalspindle 81. The ball bearing 83 is retained against a shoulder 101formed on the vertical spindle 81. The screw 98 is removable, and a.Water-tight seal 102 is provided in the outer vertical tube 95. As isapparent, the outer vertical tube 95 is stationary, while the innervertical tube 94 may pivot or rotate in unison with the propeller 86 andthe stabilizer 91. This pivoting action may be achieved simultaneouslywith the drive being transmitted to the propeller 23, and the means forfacilitating a pivoting of the stabilizer 91 (which is essential innavigation) will now be explained in detail.

With reference, again, to FIGURES 3 and 4, a drum or pulley 103 ispress-fitted or otherwise secured to the upper end of the inner verticaltube 94, and the pulley 103 carries a control cable 104, which iscoupled by conventional means (not shown) to the steering wheel 25 ofthe ship (see FIGURE 1). As shown more particularly in FIGURE 3, thecontrol cable 104 passes through suitable grommets 105 and 106 disposedin the lowermost side housing members 107 and 108, respectively.Moreover, each of the pulleys 103 is coupled for movement in unison bythe single control cable 104, such that the propellers 23 and 24 will bemoved or pivoted in unison. The pulley 103 is journaled upon a ballthrust bearing 109 between the pulley 103 and the lowermost housingmembers 110 and 111 of the propeller drive housing 21. Also, shims 112,illustrated in broken lines in FIGURE 4, may be interposed be- 6 V Vtween the propeller drive housing 21 and the lower deck 17 of the ship10.

It will be further appreciated, of course, that throughout the structureof the present invention, suitable lubricants are employed in a mannerknown to one skilled in the art. For example, oil cups 113 and 114(illustrated in FIGURE 4) are provided to facilitate lubrication.

Thus, a twin-screw marine drive mechanism is provided for use inconjunction with a single inboard engine, especially suitable forsmaller craft in the order of thirty feet or less, wherein all of thebasic advantages of twin-screw drive mechanisms is realized consonantwith the ownership of the smaller craft.

Obviously, many modifications may be made Without departing from thebasic spirit of the present invention, and therefore, within the scopeof the appended claims the invention may be practiced other than hasbeen specifically described.

We claim:

1. A twin-screw marine drive mechanism, comprising:

(a) an inboard engine;

(b) an inboard main drive spindle journaled for rotation in the sternportion of the ship and disposed transversely of the keel;

(c) means coupling said spindle to said engine;

(d) a pair of propeller drive housings, secured to the lower deck of thestern, and one on each side of the keel, with said spindle havingrespective end portions within said housings;

(e) a pair of substantially-vertical stationary housings, one for eachof said propeller drive housings, with said stationary housings beingsecured to said re spective drive housings, and with each of saidstationary housings depending therefrom and projecting through the lowerdeck and into the water;

( a rotatable sleeve journaled within each of said stationary housings,with each of said sleeves having an upper end portion within itsrespective propeller drive housing;

(g) an intermediate drive spindle journaled Within each of said sleeves;

(h) means in said propeller drive housings to couple said intermediatedrive spindles to said main drive spindle;

(i) a clutch mechanism in each of said last-named means; with saidclutch mechanisms being independently controllable from a single source,and with each of said clutch mechanisms having a forward, a reverse, anda neutral position;

(i) a propeller housing, formed as a stabilizer, and

mounted upon the lower end of each of said rotatable sleeves forconjoint movement therewith;

(k) a propeller spindle journaled Within each of said last-namedhousings;

(1) means coupling said propeller spindle to its respective intermediatedrive spindle;

(m) a propeller mounted upon each of said propeller spindles;

(n) slip clutch means between said propeller and its respectivepropeller spindle; (o) a control member mounted upon the upper end ofeach of said rotatable sleeves for conjoint movement therewith; and

(p) control means interconnecting said control members with each otherand with a single control source, whereby said propellers, when pivoted,are always in unison with each other, simultaneously with the drivebeing supplied to each.

2. The combination of claim 1, wherein:

(a) said control member comprises a drum mounted upon the upper end ofsaid rotatable sleeve for conjoint movement therewith;

(b) thrust bearing means between said drum and its respective propellerdrive housing; and

(c) said control means comprises a single control 7 cableinterconnecting said drums with each other and with the control source.

3. The combination of claim 1, wherein:

(a) said inboard engine has an engine shaft disposed longitudinally ofthe ship and transverse to said main drive spindle; and

(b) said means coupling said main drive spindle to said engine comprisesa cooperating Worm and Worm Wheel, one of which is carried by saidengine shaft, and the other of which is carried by said main drivespindle.

4. The combination of claim 1, wherein:

(a) said respective end portions of said main drive spindle each have apair of oppositely-facing bevel gears rotatably mounted thereon;

(b) said clutch mechanisms each having an axiallyshiftable respectiveclutch element to couple a desired one of said bevel gears to said maindrive spindle.

5. The combination of claim 4, wherein:

(a) said intermediate drive spindle carries a bevel pinion which isconstantly in mesh with said bevel gears.

6. The combination of claim 1, wherein:

(a) said means coupling said propeller spindle to its respectiveintermediate drive spindle comprises a pair of cooperating bevel gears,one of which is carried by said propeller spindle, and the other ofwhich is carried by said intermediate drive spindle.

7. The combination of claim 1, wherein:

(a) said slip clutch means comprises a resilient bushing between saidpropeller and said propeller spindle.

References Cited in the file of this patent UNITED STATES PATENTS1,300,398 Iaeger Apr. 15, 1919 1,943,288 Chandler Jan. 16, 19342,569,144 Benson Sept. 25,1951 2,569,346 Shively Sept. 25,1951 2,936,730Patty May 17, 1960

1. A TWIN-SCREW MARINE DRIVE MECHANISM, COMPRISING: (A) AN INBOARDENGINE; (B) AN INBOARD MAIN DRIVE SPINDLE JOURNALED FOR ROTATION IN THESTERN PORTION OF THE SHIP AND DISPOSED TRANSVERSELY OF THE KEEL; (C)MEANS COUPLING SAID SPINDLE TO SAID ENGINE; (D) A PAIR OF PROPELLERDRIVE HOUSINGS, SECURED TO THE LOWER DECK OF THE STERN, AND ONE ON EACHSIDE OF THE KEEL, WITH SAID SPINDLE HAVING RESPECTIVE END PORTIONSWITHIN SAID HOUSINGS; (E) A PAIR OF SUBSTANTIALLY-VERTICAL STATIONARYHOUSINGS, ONE FOR EACH OF SAID PROPELLER DRIVE HOUSINGS, WITH SAIDSTATIONARY HOUSINGS BEING SECURED TO SAID RESPECTIVE DRIVE HOUSINGS, ANDWITH EACH OF SAID STATIONARY HOUSINGS DEPENDING THEREFROM AND PROJECTINGTHROUGH THE LOWER DECK AND INTO THE WATER; (F) A ROTATABLE SLEEVEJOURNALED WITHIN EACH OF SAID STATIONARY HOUSINGS, WITH EACH OF SAIDSLEEVES HAVING AN UPPER END PORTION WITHIN ITS RESPECTIVE PROPELLERDRIVE HOUSING; (G) AN INTERMEDIATE DRIVE SPINDLE JOURNALED WITHIN EACHOF SAID SLEEVES; (H) MEANS IN SAID PROPELLER DRIVE HOUSINGS TO COUPLESAID INTERMEDIATE DRIVE SPINDLES TO SAID MAIN DRIVE SPINDLE; (I) ACLUTCH MECHANISM IN EACH OF SAID LAST-NAMED MEANS; WITH SAID CLUTCHMECHANISMS BEING INDEPENDENTLY CONTROLLABLE FROM A SINGLE SOURCE, ANDWITH EACH OF SAID CLUTCH MECHANISMS HAVING A "FORWARD," A "REVERSE," ANDA "NEUTRAL" POSITION; (J) A PROPELLER HOUSING, FORMED AS A STABILIZER,AND MOUNTED UPON THE LOWER END OF EACH OF SAID ROTATABLE SLEEVES FORCONJOINT MOVEMENT THEREWITH; (K) A PROPELLER SPINDLE JOURNALED WITHINEACH OF SAID LAST-NAMED HOUSINGS; (L) MEANS COUPLING SAID PROPELLERSPINDLE TO ITS RESPECTIVE INTERMEDIATE DRIVE SPINDLE; (M) A PROPELLERMOUNTED UPON EACH OF SAID PROPELLER SPINDLES; (N) SLIP CLUTCH MEANSBETWEEN SAID PROPELLER AND ITS RESPECTIVE PROPELLER SPINDLE; (O) ACONTROL MEMBER MOUNTED UPON THE UPPER END OF EACH OF SAID ROTATABLESLEEVES FOR CONJOINT MOVEMENT THEREWITH; AND (P) CONTROL MEANSINTERCONNECTING SAID CONTROL MEMBERS WITH EACH OTHER AND WITH A SINGLECONTROL SOURCE, WHEREBY SAID PROPELLERS, WHEN PIVOTED, ARE ALWAYS INUNISON WITH EACH OTHER, SIMULTANEOUSLY WITH THE DRIVE BEING SUPPLIED TOEACH.